1. Field of the Invention
The present invention relates to a ferroelectric capacitor comprising a ferroelectric layer including a bismuth (Bi) compound oxide of a layered crystal structure, a method of manufacturing the same and a memory cell using the same.
2. Description of the Related Art
With advances in layer formation technology, considerable research and development has been carried out on a non-volatile memory using a ferroelectric thin film. Such a non-volatile memory is a non-volatile ferroelectric random access memory (FeRAM) reprogrammable at high speed through the use of high speed polarization inversion and residual polarization of a ferroelectric thin film. It is a merit of an FeRAM that programmed data is not erased in contrast to a volatile memory whose programmed data is erased on powering off. One of materials making up such an FeRAM is a bismuth compound oxide of a layered crystal structure. Such a bismuth compound oxide is an attractive material since a fatigue phenomenon is reduced therein. The fatigue phenomenon is a reduction in residual polarization value due to repeated reprogramming, which is the greatest demerit of a PZT base material, a solid solution of PbTiO.sub.3 and PbZrO.sub.3 which has been used in the art. For an application to FeRAM, it has been reported that formation of polycrystalline thin film of bismuth compound oxide is achieved.
However, a polycrystalline thin film includes not only crystal grains whose orientations contribute to ferroelectricity but also grains that do not contribute to ferroelectricity at all depending on the direction of voltage application. It is therefore required that many crystal grains are included for stably obtaining a desirable residual polarization value (Pr) in forming an FeRAM with a polycrystalline thin film. In particular, a smaller grain size is preferred for reducing the area of thin film with an advance in formation of high density and high integration FeRAM.
A related-art polycrystalline thin film of bismuth compound oxide of a layered crystal structure consisting of bismuth, strontium (Sr), tantalum (Ta) and oxygen (O) has been achieved whose residual polarization value (2Pr) is approximately 20 .mu.C/cm.sup.2 and a mean surface area of the crystal grains is approximately 0.05 .mu.m.sup.2 as disclosed in T. Atsuki et al., Jpn. J. Appl. Phys. Vol. 34 (1995) pp. 5096-5099. However, approximately twenty grains in such a size are only included if the surface area of thin film is reduced down to nearly 1 .mu.m.sup.2. Consequently it may be difficult to achieve desired ferroelectric properties. It is thus required to further reduce the grain size. An adjustment to the formula is considered for grain size reduction since a grain size relates to the formula.
However, the formula closely relates to ferroelectric properties as well. Therefore, ferroelectric properties are sacrificed for grain size reduction through a formula adjustment. On the contrary, grain size reduction is not achieved with an improvement in ferroelectric properties. For example, in a polycrystalline thin film of bismuth compound oxide of a layered crystal structure consisting of bismuth, strontium, tantalum and oxygen, desirable ferroelectric properties are obtained when the proportion of strontium is of a value less than the stoichiometric composition by 20 percent. However, the crystal grain size increases on the contrary while ferroelectric properties improve with the strontium proportion approaches the value (T. Atsuki et al., Jpn. J. Appl. Phys. Vol. 34 (1995) pp. 5096-5099; T. Noguchi et al., Jpn. Appl. Phys. Vol. 35 (1996) pp. 4900-4904 and so on). That is, simply adjusting the formula does not allow the crystal grain size to be reduced while maintaining satisfactory ferroelectric properties. A reduction in FeRAM size is not achieved, either.